Every CRT is provided with a degaussing coil, which is a coil fixed on the CRT. A supply circuit for this coil supplies a current in the coil each time the CRT is switched on. An alternative decreasing current is supplied in the degaussing coil that produces a magnetic field whose field vector changes direction with the sign of the current. The purpose of said current is to get rid of all remnant magnetic field that may still exist inside the tube. Such remnant magnetic field would produce an unwanted deflection of the electron beam of the CRT. The magnetic field produced by the alternative current degausses the magnetic material around and inside the tube, so that no more magnetic fields from magnetic material are present in the vicinity of the tube.